A dental implant is known to make up for the loss of one or more dental element, and to consist of one or more components intended to restore both the masticatory function and the aesthetic aspect.
In general, a dental implant comprises an artificial root or fixture to be implanted surgically in the bony tissue, and a stump or abutment intended for supporting a dental prosthesis and connected to the fixture. The abutment is connected to the fixture after a preset time allowing the correct execution of an osteo-integration process by which the fixture results afterwards integrated in the bony tissue. The fixture is generally provided with an outer threading to allow it to be screwed into the bony tissue, but it can also be shaped in a different way in order to be inserted by pressure into the implant seat.
More particularly, dental implants are known in which a coupling of Morse-taper coupling is made between the fixture and the abutment: the latter exhibiting a truncated-cone shank to be forced into a corresponding installation cavity formed in the fixture. To prevent the abutment to rotate relative to the fixture, the abutment's shank may be provided with an axial appendix of polygonal cross-section to be inserted into a corresponding impression provided in the bottom of the inner cavity of the fixture which receives the abutment. The presence of said appendix makes it also possible to establish a precise positional reference of the abutment with respect to the fixture. A dental implant thus structured is disclosed in the document WO 96/26685.
The Morse-taper coupling between the two parts of the implant, that is, fixture and abutment, is activated by axial impulsive forces exerted on the abutment by means of a suitable tool. Morse tapers are known to have a taper ratio of the tapered shank and the tapered cavity substantially in the range of 19.002:1 through 20.047:1. To disconnect the two parts it is necessary to apply either an impulsive force, of an intensity equal to that for the coupling, or a static tensile force of quite greater intensity. But, since the resistance of the Morse-taper coupling to the torsional loads is decidedly lower, the dentist—in most of the cases, when it is necessary to remove the abutment for intervening on the prosthesis or modifying the therapeutic program, prefers to resort to a maneuver for rotating the abutment which, once releases from the Morse-taper coupling, can be easily extracted. However, when the abutment is of a type provided with the above said appendix having polygonal cross-section, the rotational maneuver above mentioned is in actual fact prevented, since the appendix in question is inserted into the respective seat, correspondingly shaped with polygonal cross-section, exhibited by the fixture. On the other hand, when the abutment is not provided with an appendix of polygonal cross-section type, there is no accurate reference for the position of the abutment, with respect to the fixture, during both the preparation of the prosthesis and the fitting tests of the implant on the patient.